Coupling agents for orthopedic biomaterials

ABSTRACT

The invention provides a method for the preparation of bone-polymer composites wherein the mineral portion of the bone is treated with a coupling agent before being incorporated into a biocompatible polymeric matrix. The resulting composites may be used as such or be further processed to form an osteoimplant.

This application is a continuation of and claims the priority of U.S.Provisional Application No. 60/416,904, filed Oct. 8, 2002, the entirecontents of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

The importance of orthopedic substitutes is underscored by the fact thatthe World Health Authority has decreed 2000-2010 the Bone and JointDecade. Bone substitutes are the most common implanted materials, andare second only to transfused blood products as products deliveredinternally. Bone substitutes are used to help repair or replace skeletaldeficiencies resulting from trauma, tumors, surgery, congenital anddegenerative diseases or abnormal development.

Current methods for the repair of bony defects include autografting andallografting. Autologous bone grafts utilize cortical and cancellousbone that is harvested and transplanted within the same patient.Autologous bone grafting is currently the most effective procedure forrepair of bony defects, and is the standard against which all othermethods are judged. The advantages of autologous bone grafts includetheir excellent success rate, low risk of transmitting disease, andhistocompatibility. Allografts utilize bone harvested from a differentorganism of the same species, and lack the osteogenic properties ofautografts. Their healing capacity is consequently lower. Allograftingalso carries a risk of transmitting certain diseases, and may elicitintense immunological reactions. Although both autologous and allogenicgrafts can be used successfully, they suffer from problems associatedwith harvesting costs, limited availability, and donor site morbidity.

Purified and synthetic materials, including metals, plastics, ceramics,and collagen-based matrices have been developed as bone substitutes inan attempt to obviate these problems. These materials can be produced inlarge quantities and in a variety of shapes and sizes, and most arenon-immunogenic. However, metals and plastics, which were the firstsynthetic materials to be used clinically, are subject to fatigue,fracture, and wear, and do not remodel or resorb with time. Morerecently, the FDA has approved a coral derived hydroxyapatite for use incontained bone defects, and a purified collagen/ceramic compositematerial for use in acute long bone fractures. Although these materialsavoid the morbidity involved in harvesting bone and eliminate theproblems associated with limited donor bone availability, they are muchless effective than autografts. This explains, at least partly, the factthat in 2000 synthetic bone substitutes represented less than 15% of theglobal use of bone grafts.

There is, therefore, a continued interest in the development of new,improved bone graft materials. Knowledge of the structure and mechanicalproperties of bone and a better understanding of the natural bonehealing process have allowed investigators to define desirablecharacteristics of a successful implant material. Bone substitutesshould desirably be biocompatible, osteoconductive, integrative andmechanically compatible with native bone. Materials that areosteoinductive are particularly desirable. These materials shouldprovide cell anchorage sites, mechanical stability and structuralguidance, and serve as a source of osteogenesis over the time periodrequired for bone replacement.

Since the biological and mechanical properties of bone result from itsmicrostructural features, a strategy in the development of the idealsubstitute material is to mimic the structure of natural bone. Bone is acomposite material made up of organic and inorganic components, wherethe inorganic or mineral phase represents 60-70% of the total dry boneweight. The organic phase is a viscous gel-like material comprisedprimarily of type I collagen while the mineral component consists of acrystalline form of calcium phosphate containing carbonate ions, smallamounts of sodium, magnesium, hydrogenophosphate ions and other traceelements. The interaction of the hard brittle mineral phase and theflexible organic matrix gives bone its unique mechanical properties. Theability of bone to perpetually remodel is ascribed, at least in part, tothe calcium phosphate ratio of the mineral phase as well as to theparticular crystalline nature of bone. A sound approach in developing abone substitute is therefore to combine minerals to an organic polymericmatrix to generate a composite material exhibiting the toughness andflexibility of the polymer and the strength and hardness of the mineralfiller.

In recent years, several of these composites have been designed anddeveloped, with powders or ceramics of calcium phosphate (the main bonemineral component) acting as inorganic fillers. Among the calciumphosphate ceramics, hydroxyapatite and tricalcium phosphate ceramics arethe most commonly used. Calcium phosphate-based composites possessunique advantages over their constituents, combining theosteoconductivity of the mineral with the easy processing of polymers.In addition, by taking advantage of the wide range of properties ofpolymers, composites can be made to meet the needs of a large variety ofclinical applications. Numerous patents disclose the preparation andcomposition of such bone substitutes made of calcium phosphate andnatural (U.S. Pat. Nos. 4,516,276; 4,776,890; 5,626,861; 6,201,039; and6,395,036) or synthetic (U.S. Pat. Nos. 4,192,021; 4,263,185; 4,187,852;and 5,338,772) polymers.

Another series of composites, based on the use of bone particles asmineral fillers, has also been developed. Most of these compositematerials are prepared from demineralized bone (from human or animalorigin) and biocompatible polymers (see, for example, U.S. Pat. Nos.4,394,370; 4,440,750; 4,863,732; and 5,531,791). The demineralizationprocess is carried out to totally or partially remove minerals andbetter expose the bone collagen in order to favor the binding of thebone particles to the organic polymer matrix. The resulting compositionscan be delivered in a fluid or gel state, they promote cellularinfiltration from adjacent osseous tissues, and may possessosteoinductive and osteoconductive properties. Implantable sponges,bandages or prostheses have been formed from these demineralizedbone/collagen composites (U.S. Pat. No. 4,394,370).

However, demineralized materials are rarely employed as load-bearingbone products, which are used at implant sites where the bone graft isexpected to withstand some level of physical load. Several attempts havebeen made to produce materials with mechanical properties as close aspossible to those of natural bone. Some preparation methods discloseremoving all organic material from bone to yield bone mineral bypyrolytic or chemical processes (U.S. Pat. No. 4,882,149) or by using afluid in the supercritical state (U.S. Pat. No. 6,217,614). Otherprocedures advocate the removal of only part of the organic component(in U.S. Pat. No. 6,261,586, for example, the bone material is processedto remove associated non-collagenous bone proteins but naturallyassociated native collagen materials and bone minerals are preserved).Composites have been formed by combination of these nondemineralizedbone materials with natural polymers, such as collagen and gelatin (U.S.Pat. Nos. 4,314,380 and 5,573,771) and synthetic polymers, such aslactic polyester (U.S. Pat. No. 5,573,771). Most of these products areintended to be used as remodeling implants, vertebral spacers orprosthetic bone replacements.

Although the composite materials described above have led to theproduction of biocompatible load-bearing implants with attractivecharacteristics, they are still in need of improvement. Actually, noneof the calcium phosphate-based composites have been shown to possess invivo mechanical properties comparable to those of natural bone and inmost cases, the same is true for the bone-composite materials. Ingeneral, these composites exhibit a poor polymer/filler interface [Reiset al. “Structure development and control of injection-mouldedhydroxyapatite-reinforced starch/EVOH composites” Adv. Polym. Tech.16:263-277 (1997)]. In the absence of a good interfacial adhesionbetween the organic polymer and the mineral filler, transfer of thestresses experienced by the load-bearing implant from the “soft” polymerto the “hard” filler is difficult. A lack of adhesion between the twophases results in early failure. In the case of industrial composites,the compatibility between the filler and the polymer has long been knownto improve by using several types of surface coatings, coupling agents,or other additives.

In the field of biomaterials, similar methods have recently been appliedto improve the interface of hydroxyapatite/polymer composites usingcoupling agents [Nishizawa et al. “Surface modification of calciumphosphate ceramics with silane coupling agents” Chem. Soc. Jpn. 1:63-67(1995); Dupraz et al. “Characterization of silane-treated hydroxyapatitepowders for use as filler in biodegradable composites” J. Biomed. Mater.Res. 30:231-238 (1996)]; zirconium salts [Misra, “Adsorption ofzirconium salts and their acids in hydroxyapatite: The use of salts ascoupling agents to dental polymer composites” J. Dent. Res. 12:1405-1408(1985)]; and polyacids [Liu et al. “Surface modification ofhydroxyapatite to introduce interfacial bonding with Polyactive™ 70/30in a biodegradable composite” J. Mater. Sci. Mater. Med. 7:551-557(1996); and Liu et al. “Polyacids as bonding agents inhydroxyapatite/polyester-ether Polyactive™ 30/70 composites” J. Mater.Sci. Mater. Med. 9:23-30 (1998)]. For the same purpose, hydroxyethylmethacrylate has been chemically coupled to octocalcium phosphate[Delpech et al. “Calcium phosphate and interfaces in orthopedic cements”Clin. Mater. 5:209-216 (1990); and Dandurand et al. “Study of themineral-organic linkage in an apatitic-reinforced bone cement” J.Biomed. Mater. Res. 24:1377-1384 (1990)], and polyethylene glycol hasbeen grafted to the surface of nano-apatite [Liu et al. “Covalentbonding of PMMA, PBMA, and poly(HEMA) to hydroxyapatite particles” J.Biomed. Mater. Res. 40: 257-263 (1998)]. U.S. Pat. No. 6,399,693discloses a composite material comprising a mixture of silanefunctionalized polyaromatic polymer and an organic or inorganic materialcontaining moieties reactive with the silane groups. In most cases,these treatments result in significant improvements in the ultimatestiffness of the composite. However, one major drawback lies in the factthat, in the presence of the different coupling agents and additives,the chemical bonds formed between hydroxyapatite and the polymer matrixare too “permanent” (i.e., they are too strong and too stable tohydrolysis, dissolution, and/or biological/enzymatic attack) therebyinhibiting the remodeling of the grafting material and gradualdegradation of the composite.

SUMMARY OF THE INVENTION

The disadvantages of the prior art are overcome by the presentinvention, which provides a system for producing composite materials bybinding a biocompatible organic polymeric matrix to the mineral portionof bone using coupling agents.

The invention provides composite materials that are useful as bonesubstitutes for weight-bearing purposes, exhibit improved mechanicalproperties as a result of enhanced interfacial stability, and, unlikethe hydroxyapatite/polymer composites described above, are also able togradually transfer the initial load to the host bone tissue as theyundergo remodeling and degradation.

Natural bone has been shown to be not as structurally close tohydroxyapatite (the chemical formula of which is: Ca₅(PO₄)₃OH) as wasoriginally believed. For example, in addition to calcium phosphate,natural bone is also made of carbonate ions, magnesium, sodium,hydrogenophosphate ions and trace elements. There is also evidence that,unlike hydroxyapatite, bone crystals contain only a few hydroxyl groups[Bonar et al. “Structural and composition studies on the mineral ofnewly formed dental enamel: A chemical, x-ray diffraction, and ³¹P andproton nuclear magnetic resonance study” J. Bone Min. Res. 6:1197-1176(1991)]. Moreover, many of the carbonate (CO₃ ²⁻) and hydrogenophosphate(HPO₄ ²⁻) groups in bone crystals are, from the structural and chemicalpoints of view, unstable and very reactive, thus providing certainphysical, chemical and biological functional features important in theformation and dissolution of the crystals in biological tissues. Inaddition, the short-range environment of the HPO₄ ²⁻ groups in bonecrystals has been shown to be distinctly different from that of the HPO₄²⁻ groups in synthetic apatites and other related calcium phosphatecrystals [Wu, Ph.D. thesis MIT, “Solid state NMR study of bone mineral”,August 1992]. These differences between bone crystals and synthetichydroxyapatite result in significant differences in their reactivity,biodegradability and remodeling ability in vivo.

The chemical bonds created between the mineral portion of bone and apolymer are thus weaker and less stable (to hydrolysis, dissolution,and/or biological/enzymatic attack) than those formed betweenhydroxyapatite and a polymeric matrix. Consequently, in the case ofbone-derived composites, the use of coupling agents described in thisinvention leads to a better interfacial adhesion and therefore to morefavorable mechanical properties without causing the problems associatedwith inhibition of remodeling and biodegradability that arise whenhydroxyapatite serves as mineral filler. The fact that the mechanicalstrength of the bone-polymer composites is improved (only) for the timeperiod required for the bone healing process to be completed constitutesone of the major advantages of the present invention.

The invention also provides preparation methods that allow control overthe chemical strength and biological/chemical/enzymatic stability of thebonds formed between the mineral portion of bone and the organic matrix.More specifically, the invention provides strategies for weakening orstrengthening the chemical bonds. The stability of these bonds can bereduced by modifying either the mineral phase or the polymeric matrixphase, or both. These modifications, carried out before incorporation ofthe bone particles into the polymer, make these materials lesschemically and biologically stable. One way to modify the bone surfaceis to recrystallize it in order to generate a more soluble mineralcomposition. This can be accomplished, for example, by treating the bonesurface with dilute phosphoric acid, which substantially transforms theapatite to dicalcium phosphate dihydrate. The modified bone particlesend up being coupled to the polymer matrix through the less stabledicalcium phosphate crystals. On the other hand, methods can be used tostrengthen the polymer/bone particles bonds by improving the binding ofthe coupling agent to the polymer matrix. Several strategies exist: thecoupling agent can be optimized to efficiently bind to the matrixmaterial used, polymers can be chemically modified and made morereactive to the coupling agent used, or a cross-linking agent can beadded to the composite to help the binding of the coupling agent to thepolymer. All these methods make it easier to prepare bone-derivedcomposites with controlled, predictable stability and with mechanicalproperties that can be tailored to meet the needs and requirements oftheir clinical applications.

In the systems described in this invention, the bone particles can be ofautologous, allogenic or xenogenic origin, prepared from cortical bone,cancellous bone, or cortico-cancellous bone, and can benondemineralized, deorganified or anorganic. The organic matrices arepreferably biocompatible polymers and, if desired, degradablebiocompatible polymers. They can be of natural or synthetic origin, orany combination of natural and synthetic polymers. In one embodiment,the coupling agents are silane compounds. The incorporation of boneparticles into the polymer matrix can be achieved using one or acombination of fabrication techniques known to those skilled in the art.

Another aspect of the invention concerns composites formed by reactingbone particles with coupling agents before incorporation into abiocompatible organic polymer. The polymeric matrix and/or the surfaceof the bone particles can be optionally modified beforehand. The finalproducts made using these composites can be formed by molding, casting,machining, vacuum forming, or any fabrication technique or combinationof fabrication techniques known in the art. The constructs containingthe nondemineralized bone particles are preferentially weight bearingand are able to initially support load and gradually transfer this loadto the host bone tissue as it remodels the implants.

In another aspect, the invention provides bone substitute materials thatcan be used for non load-bearing purposes. For example, a flowablematerial for filling defects in cancellous bone such as vertebral bodiesmight benefit from a strong interaction between the bone particles andthe fluid or gel phase. This can prevent settling and improve thelubrication and flow properties of the material in order to makeinjection easier.

Another important advantage of the composites described in thisinvention lies in their ability to function as a carrier for, andeffectively incorporate, one or more medically/surgically usefulsubstances. For example, these substances can promote new bone growthand connective tissue regeneration, and/or accelerate wound healing.

The present invention also provides a method for binding an organicpolymeric matrix, through the use of coupling agents, to constructs thatincorporate large pieces of bone. For example, the coupling agent can beused to bond a polymeric surface coating to a monolithic bone piece orto bond several columns of bone together to form a weight bearingimplant.

DEFINITIONS

The term osteogenic, as used herein, refers to the ability of asubstance or material to induce new bone formation via the participationof living cells from within the substance.

The term osteoconductive, as used herein, refers to the ability of asubstance or material to provide biologically inert surfaces which arereceptive to the growth of new host bone.

The term osteoinductive as used herein, refers to the ability of asubstance or material to recruit cells from the host that have thepotential for repairing the bone tissue.

The term osteoimplant is herein used in its broadest sense and is notintended to be limited to any particular shapes, sizes, configurationsor applications.

Mechanical strength as used herein, refers to those properties exhibitedby a bone graft, including loading strength, compressive strength, andtensile strength.

The terms load bearing or weight bearing as used herein, refer to a boneproduct for implantation in a patient at a site where the bone graft isexpected to withstand some level of physical load. The terms “loadbearing” and “weight bearing” are herein used interchangeably.

Nondemineralized, as herein applied to bone particles, refers to boneparticles that were not subjected to a demineralization process (i.e., aprocedure that totally or partially removes the original inorganiccontent of bone).

Demineralized, as herein applied to bone particles, refers to boneparticles that were subjected to a demineralization process (i.e., aprocedure that totally or partially removes the original inorganiccontent of bone).

Deorganified, as herein applied to bone particles, refers to boneparticles that were subjected to a process that removes part of theiroriginal organic content.

Anorganic, as herein applied to bone particles, refers to bone particlesthat were subjected to a process that removes their entire originalorganic content.

The term biocompatible, as used herein, is intended to describematerials that upon administration in vivo, do not induce undesirablelong-term effects.

Biodegradable, as used herein, refers to the characteristic thatmaterials will degrade under physiological conditions to form a productthat can be metabolized or excreted without damage to organs.Biodegradable materials are not necessarily hydrolytically degradableand may require enzymatic action to fully degrade. Biodegradablematerials also include materials that are broken down by or withincells.

The term coupling agent, as used herein, refers to reagents that linkthe mineral portion of bone to the organic polymeric matrix.

A cross-linking agent, as used herein, is a compound that promotes theformation of a covalent bond between the polymer matrix and the boneparticles through a coupling agent.

The term biomolecules, as used herein, refers to the classes ofmolecules (e.g., proteins, amino acids, peptides, polynucleotides,nucleotides, carbohydrates, sugars, lipids, glycoproteins,nucleoproteins, lipoproteins, steroids, etc) that are commonly found incells or tissues, whether the molecules themselves arenaturally-occurring or artificially created (e.g., by synthetic orrecombinant methods). For example, biomolecules include, but are notlimited to, enzymes, receptors, neurotransmitters, hormones, cytokines,cell response modifiers such as growth factors and chemotactic factors,antibodies, vaccines, haptens, toxins, interferons, ribozymes,anti-sense agents, plasmids, DNA, and RNA.

The terms polynucleotide, nucleic acid, and oligonucleotide refer topolymers of nucleotides. These terms can be used interchangeably.Typically, a polynucleotide comprises at least three nucleosides (i.e.,adenosine, thymidine, guanosine, cytidine, uridine, deoxyadenosine,deoxythymidine, deoxyguanosine, and deoxycytidine), nucleoside analogs(e.g., 2-amino-adenosine, 2-thiothymidine, inosine, pyrrolopyrimidine,3-methyl adenosine, C5-pronynyl-cytidine, C5-propynyluridine,C5-bromouridine, C5-fluorouridine, C5-iodouridine, C5-methyl-cytidine,7-deazaadenosine, 7-deazaguanosine, 8-oxoadenosine, 8-oxoguanosine,O(6)-methyl-guanine, and 2-thiocytidine), chemically modified bases,biologically modified bases (e.g., methylated bases), intercalatedbases, modified sugars (e.g., 2′-fluororibose, ribose, 2′-deoxy-ribose,arabinose, and hexose), or modified phosphate groups (e.g.,phosphorothioates and 5′-N-phosphoramidite linkages).

A polypeptide, peptide or protein comprises a string of at least threeamino acids linked together by peptide bonds. The terms “polypeptide”,“peptide” and “protein” may be used interchangeably. Peptide may referto an individual peptide or a collection of peptides. Inventive peptidespreferably contain only natural amino acids, although non-naturalamino-acids (i.e., compounds that do not occur in nature but that can beincorporated into a polypeptide chain) and/or amino acid analogs as areknown in the art may alternatively be employed. Also, one or more of theamino acids in a peptide may be modified, for example, by the additionof a chemical entity such as a carbohydrate group, a phosphate group, afarnesyl group, an isofarnesyl group, a fatty acid group, a linker forconjugation, functionalization, or other modification. In a preferredembodiment, the modifications of the peptide lead to a more stablepeptide (e.g., greater half-life in vivo). These modifications mayinclude cyclization of the peptide, the incorporation of D-amino-acids,etc. None of the modifications should substantially interfere with thedesired biological activity of the peptide.

The terms polysaccharide, carbohydrate and oligosaccharide refer to apolymer of sugars. Typically, a polysaccharide comprises at least threesugars. The polymer may include natural sugars (e.g., glucose, fructose,galactose, mannose, arabinose, ribose, and xylose) and/or modifiedsugars (e.g., 2′-fluororibose, 2′-deoxyribose, and hexose). The terms“polysaccharide”, “carbohydrate” and “oligosaccharide” may be usedinterchangeably.

The term small molecule is used to refer to molecules, whethernaturally-occurring or artificially created (e.g., via chemicalsynthesis) that have a relatively low molecular weight. Typically, smallmolecules are monomeric and have a molecular weight of less than about1500 g/mol. Preferred small molecules are biologically active in thatthey produce a local or systemic effect in the patient. In certainpreferred embodiments, the small molecule is a drug. Preferably, thoughnot necessarily, the drug is one that has already been deemed safe andeffective for use by the appropriate governmental agency or body. Forexample, drugs for human use listed by the FDA under 21 C.F.R. §§ 330.5,331 through 361, and 440 through 460; drugs for veterinary use listed bythe FDA under 21 C.F.R. §§ 500 through 589, incorporated herein byreference, are all considered acceptable for use in accordance to thepresent invention.

The term bioactive agents, as used herein, refers to compounds orentities that alter, inhibit, activate, or otherwise affect biologicalor chemical events. For example, bioactive agents include, but are notlimited to, anti-AIDS substances, anti-cancer substances, antibiotics,immunosuppressants, anti-viral substances, enzyme inhibitors,neurotoxins, opioids, hypnotics, anti-histamines, lubricants,tranquilizers, anti-convulsants, muscle relaxants and anti-Parkinsonsubstances, anti-spasmodics and muscle contractants including channelblockers, miotics and anti-cholinergics, anti-glaucoma compounds,anti-parasite and/or anti-protozoal compounds, modulators ofcell-extracellular matrix interactions including cell growth inhibitorsand anti-adhesion molecules, vasodilating agents, inhibitors of DNA, RNAor protein synthesis, anti-hypertensives, analgesics, anti-pyretics,steroidal and non-steroidal anti-inflammatory agents, anti-angiogenicfactors, anti-secretory factors, anticoagulants and/or anti-thromboticagents, local anesthetics, ophthalmics, prostaglandins,anti-depressants, anti-psychotic substances, anti-emetics, and imagingagents. In certain preferred embodiments, the bioactive agent is a drug.

A more complete listing of bioactive agents and specific drugs suitablefor use in the present invention may be found in “PharmaceuticalSubstances: Syntheses, Patents, Applications” by Axel Kleemann and JugenEngel, Thieme Medical Publishing, 1999; the “Merk Index: An Encyclopediaof Chemicals, Drugs, and Biologicals”, Edited by Susan Budavri et al.,CRC Press, 1996, and the United States Pharmacopeia-25/NationalFormulary-20, published by the United States Pharmacopeial Convention,Inc., Rockville Md., 2001, all of which are incorporated herein byreference.

A targeting agent, as used herein, is any chemical entity which, whenincluded in an inventive composite, will direct the composite to aparticular site or cause the inventive composite to remain in aparticular site within the recipient's body. A targeting agent may be asmall molecule, peptide, protein, biological molecule, polynucleotide,etc. Typical targeting agents are antibodies, ligands of knownreceptors, and receptors.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

The present invention provides a bone-polymer composite for use inorthopedic medicine, where it may serve as a bone substitute material,or provide a convenient source of bone-derived particles for producingweight bearing implants. Preferred inventive composites are materialsthat are biocompatible, display strength throughout the bone repair andremodeling process, and resorb gradually. In particular, the inventionprovides a composite that is made by bonding a biocompatible polymer tothe mineral portion of bone particles using a coupling agent.

Certain aspects of preferred embodiments of the invention are describedbelow in more detail. Those of ordinary skill will appreciate that avariety of embodiments or versions of the invention are not specificallydiscussed but are nonetheless within the scope of the present invention,as defined by the appended claims.

Bone Particles

The bone particles employed in the preparation of the bone/polymercomposite of the invention can be obtained from cortical, cancellousand/or cortico-cancellous bone which may be of autogenous, allogenicand/or xenogenic origin. However, it is preferred that the source of thebone be matched to the eventual recipient of the inventive composition(i.e., the donor and recipient should, at least, be of the samespecies).

Preparation of Bone Particles. Methods of preparation of bone particlesare known in the art. Bone particles can be formed by milling whole boneto produce fibers, chipping whole bone, cutting whole bone, fracturingwhole bone in liquid nitrogen, or otherwise disintegrating the bonetissue. Particles can optionally be sieved to produce those of aspecific size. The bone particles employed in the inventive compositecan be powdered bone particles possessing a wide range of particlessizes ranging from relatively fine powder to coarse grains and evenlarge chips. In one embodiment, powdered bone particles can range inaverage particle size from about 0.05 to about 1.2 mm and possess anaverage median length to median thickness ratio of from about 1:1 toabout 3:1. If desired, powdered bone particles can be graded intodifferent sizes to reduce or eliminate any less desirable size(s) ofparticles that may be present.

Alternatively, or in combination with the aforementioned bone powder,elongate bone particles (that exhibit a high median length to medianthickness ratio) can be used. In overall appearance, elongate boneparticles can be described as filaments, fibers, threads, slender ornarrow strips, etc. Such elongate particles can be obtained by any oneof several methods, e.g., by milling or shaving the surface of an entireor relatively large section of bone. Employing a milling technique, onecan obtain a mass of elongated bone particles containing, for example,at least about 60 weight percent of elongate bone particles possessing amedian length of about 2 to about 200 mm or more, a median thickness offrom about 0.05 to about 2 mm, and a median width of from about 1 mm toabout 20 mm. Such elongate bone particles can possess a median length tomedian thickness ratio of at least about 50:1 up to about 500:1 or more,and a median length to median width ratio of from about 10:1 to about200:1. The milling process may be optimized to adjust the size of thebone particles and the size distribution.

Another procedure for obtaining elongate bone particles, particularlyuseful for pieces of bone up to about 100 mm in length, is the boneprocessing mill described in commonly assigned U.S. Pat. No. 5,607,269.Use of this bone mill results in the production of long, thin stripsthat quickly curl lengthwise to provide tubular-like bone particles. Ifdesired, elongate bone particles can be graded into different sizes toreduce or eliminate any less desirable size(s) that may be present.

The composite of the invention can be made using particulate boneparticles, or elongated bone particles or a mixture of both. In thelatter case, the mechanical properties of the final composite can betailored by adjusting the weight percent of the various shapes (elongateor particulate) of bone particles.

Modification of the Components of Bone Particles. Bone particles used inthe fabrication of the inventive composites can be nondemineralized,deorganified or anorganic.

When used in a composite, nondemineralized bone particles play a dualrole. They act as a stiffener, providing strength and enhancing theability to support load, and also bring about new bone ingrowth byosteoinduction. Thus, as the healing process progresses over time, thesebone particles are gradually remodeled and replaced by new host bone.The use of nondemineralized bone particles is highly preferred, but notessential, in the fabrication of the composite of the present invention.

Bones particles can be subjected to a process that partially or totallyremoves their initial organic content to yield deorganified andanorganic bone particles, respectively. Different mineralization methodshave been developed and are known to those skilled in the art [Hurley etal. “Anorganic bone—chemistry, anatomy, and biological reactions” Milit.Med. 101-104 (1957); Kershaw “Preparation of anorganic bone graftingmaterial” Pharm. J. 8:537 (1963); and U.S. Pat. No. 4,882,149]. Thepreferred mineralization procedure includes a de-greasing step followedby a basic treatment (with ammonia or an amine) to degrade residualprotein and an extensive water washing (U.S. Pat. Nos. 5,417,975 and5,573,771). Deorganified and anorganic bone particles are used in thecomposite of the invention when, for example, the presence of organicmaterial residues could lead to undesirable immunological response onimplantation.

Other exemplary modifications include removing water, e.g., by drying orlyophilization, and reducing or removing lipids by a defatting process.Defatting may be accomplished using lipase enzymes or washing with achloroform methanol mixture or by washing in alcohols such as methanol,ethanol or isopropanol. Some form of energy may be provided duringwashing, for example, through heat, ultrasonic agitation, or applicationof a pressure gradient. For example, U.S. Pat. No. 5,846,484 disclosesmethods of using pressure to move fluid from the endosteal portion ofbone to the periosteal portion of bone through the vasculature.Essentially, one portion of the bone is placed in a pressure chamber,where fluid is forced into the bone. The fluid passes into the medullarycanal and is forced out of the portion of the bone that is outside ofthe pressure chamber through the vasculature. Application of vacuumallows the process to be run in the reverse direction. One skilled inthe art will understand that alternative methods of removing fats orwater may also be exploited, including alternative methods of exploitinga pressure gradient to infiltrate bone with a fluid.

Alternatively or in addition, the bone particles may be treated with adetergent, surfactant, or solvent, or pathogens within the boneparticles may be removed or inactivated. Exemplary pathogens are wellknown to those skilled in the art and include bacteria, spores, mold,fungi, and viruses. Methods of removing and/or inactivating thesepathogens are well known to those skilled in the art and include forexample, radiation sterilization, antibiotic treatment, and treatmentwith pathogen inactivating chemicals.

Modification of the Surface of Bone Particles. Optionally, the boneparticles used in the preparation of the inventive composite can bemodified on their surfaces. In one embodiment, the bone particle surfaceis chemically treated before being derivatized with a coupling agent.One way to modify the mineral phase is to recrystallize the surface toform a more soluble mineral composition. For example, nondemineralizedbone particles may be rinsed with dilute phosphoric acid (e.g., for 1 to15 minutes in a 5-50% solution by volume). Phosphoric acid reacts withthe mineral component of the bone and coats the particles with dicalciumphosphate dihydrate. The latter inorganic compound is more soluble thannon-treated bone mineral and therefore forms less stable bonds with thecoupling agent.

Coupling Agents

One factor that determines the final performance of a composite materialis the quality of the filler/polymer interface. In recent years, thereinforcing power of mineral fillers has been improved by a chemicaltreatment that links the two components of a composite by covalent bondsusing coupling agents. Commonly used coupling agents include silanes,zirconates, and titanates. A wide variety of these coupling agents arecommercially available from different manufacturers.

Silane Coupling Agents. In a preferred embodiment, silane couplingagents are employed to act as mediators and bind a biocompatible organicpolymer to the mineral portion of bone particles.

An organosilane molecule has the general chemical formula:R_(n)SiR′_(m)where m is a whole number between 1 and (4-n). A silane coupling agentexhibits three main constituents: a silicon atom (Si), which is attachedto R, a non hydrolyzable organic functional group (e.g., vinyl, epoxy,amino, methacryl, acryl, isocyanato, thiocyanato, mercapto, chloro, etc)and to R′ a hydrolyzable or good leaving functional group (e.g.,acetoxy, alkoxy, chloro, hydride, etc). The R′ group is involved in thereaction with the inorganic material (mineral filler), while R possessesa functionality which enables the coupling agent to bind covalently toan organic substance (polymeric matrix). Most of the widely used silaneshave one organic substituent (i.e., n=1). In most cases, the silane issubjected to hydrolysis prior to the coupling reaction. The highlyreactive silanol groups, that are generated by hydrolysis, subsequentlyform metal hydroxyde or silaxone bonds with the inorganic material.Regardless of the value of m (i.e., m=1, 2 or 3 if n=1), there isusually only one bond formed between the silane and the mineralsubstrate; the other silanol groups (if present, i.e., if m=2 or 3)exist as either bonded to the silicon atoms of other coupling moleculesor in free forms.

Exemplary silanes include: 3-methacryloxypropyltrimethoxysilane,3-aminopropyl-trimethoxysilane, 3-aminopropyltriethoxysilane,3-glycidoxypropyltrimethoxysilane, trimethoxy-vinylsilane, andpoly(vinylmethoxysiloxane)

Selection of a Silane Coupling Agent. Selection of the appropriatecoupling agent is accomplished by empirical evaluation of silanes withinpredicted categories. Exact prediction of the best silane can becomplicated as an increase in interfacial adhesion via the use ofsilanes is the result of a complex series of factors (such as surfaceenergy, polar adsorption, acid-base interaction, etc). Strategies foroptimization must take into account the materials on both sides of theinterface (i.e., the mineral portion of bone particles and the organicpolymer matrix) and their susceptibilities to the various couplingfactors.

The number of R′ groups on the silane is another important parameter incontrolling bond characteristics. The traditional silane coupling agentscontain three hydrolyzable or leaving groups (i.e., m=3). These couplingmolecules have maximum hydrolytic stability but tend to be hydroscopic.Silanes with two R′ groups form less rigid interfaces whereas silaneswith only one leaving group yield the most hydrophobic interfaces buthave the lowest hydrolytic stability.

In the silane molecule, the silicon atom and the functional group R canbe connected by an elongated tether group. Once the silane is attachedto the mineral portion of the bone, the tether acts as a spacer betweenthe bone particle and the terminal active group at the other end of thesilane molecule. The presence of this tether, which creates somephysical distance and thereby reduces steric hindrance, helps make theactive R group more accessible to the polymer.

Coupling Reaction. The coupling reaction can be carried out usingdifferent methods known in the art: deposition from aqueous alcohol anddeposition from aqueous solutions are the procedures most commonly usedfor preparing silylated surfaces, whereas bulk deposition onto powders,and integral blend methods, are processes that are more useful in theformulation of composites.

Modification of Silane Coupling Agents. When the commercially availablesilane coupling agents do not bear appropriate terminal functionalgroups that match, at one end, the chemical reactivity of the mineralportion of bone and, at the other end, the chemical reactivity of theorganic polymer matrix, the silane molecule can be modified. Once thecoupling agent is attached to the bone particles, its R functional groupcan be submitted to a large number of chemical reactions. These chemicalmodifications can also be carried out before reaction between the silaneand the bone particles. One skilled in the art will readily recognizehow to modify R groups such as amino, alkoxy, ketones, aromaticmoieties, etc.

In addition, the silane can be used to attach a biologically activecompound, such as a biomolecule, a small molecule or a bioactive agent,to the bone particles before their incorporation into the polymericmatrix (as described below). The silane can be optimized for thespecific compound to be associated with the bone particles. In oneembodiment, the composite of the invention can be made using silanesbearing different terminal groups R with functionalities to match thechemical reactivity of the organic polymeric matrix and that of thedifferent biologically active compounds to be incorporated into thecomposite. Similarly, non biologically active substances can be attachedto the bone particles through the silane coupling agent (see below).

Cross-Linking Agents. Another way to favor the formation of a covalentbond between the silane molecule and the organic polymer matrix is touse cross-linking agents. A large number of chemical cross-linkingagents are known to those skilled in the art. In a preferred embodiment,the cross-linkers used in the preparation of the inventive compositesare biocompatible heterobifunctional molecules.

A wide variety of heterobifuntional cross-linkers are known in the art.These include, but are not limited to, N-hydroxysuccinimide derivativesand their water soluble analogs: N-hydroxysulfosuccinimide derivatives,carbodiimide derivatives, as well as derivatives of aldehydes, epoxycompounds, polyvalent metallic oxides, organic tannins, maleimides,sulfides, phenolic oxides, hydrazide, isocyanates, thioisocyanates, etc.

Polymers

Suitable polymers useful for the preparation of the inventive compositesare preferably biocompatible polymers, that can be of natural orsynthetic origin or a combination of natural and synthetic polymers.

Natural polymers include polysaccharides and proteins. Exemplarypolysaccharides include starches, dextrans, and celluloses; exemplaryproteins include collagen and gelatin. Polysaccharides such as starches,dextrans, and celluloses may be unmodified or may be modified physicallyor chemically to affect one or more of their properties such as theircharacteristics in the hydrated state, their solubility, or theirhalf-life in vivo. An exemplary modified polysaccharide is ethylcellulose.

In one embodiment, the organic matrices are biocompatible, degradablepolymers. These polymers can be broken down by cellular action or/and byaction of non-living body fluid components. A variety of biocompatible,degradable polymers can be used. These include, but are not limited to,polyanhydrides, polyesters, polyorthoesters, poly(propylene fumerates),polyglyconates, poly(hydroxy acids), polyphosphazenes, biodegradablepolycyanoacrylates, polycaprolactones, poly(vinyl pyrrolidones),polyamides, polyurethanes, polyesteramides, polydioxanones, polyacetals,polyketals, polycarbonates, polysulfones, polyorthocarbonates,polyhydroxybutyrates (e.g., poly(3-hydroxybutyric acid)),polyhydroxyvalerates, polyalkylene oxalates, polyalkylene succinates,poly(maleic acid), poly(amino acids), poly(methyl vinyl ether),poly(maleic anhydride), tyrosine based polymers, including but notlimited to polycarbonates and polyarylates (Pulapura, et al.,Biopolymers, 1992, 32: 411-417; and Hooper, et al., J. Bioactive andCompatible Polymers, 1995, 10:327-340), Polysorb™ (available from IgusInc., Providence, R.I.), chitin, chitosan, and copolymers, terpolymers,or higher poly-monomer polymers thereof or combinations or mixturesthereof. Examples include poly(glycolide lactide-co-lactide), starchethylene vinyl alcohol, poly(3-hydroxybutyric acid-co-3-hydroxy-valericacid), and starch cellulose acetate. Non-biodegradable polymers may alsobe used as well. For example, polypyrrole, polyanilines, polythiophene,and derivatives thereof are useful electroactive polymers that cantransmit voltage from the endogenous bone to an implant. Othernon-biodegradable, yet biocompatible polymers include polystyrene,non-biodegradable polyurethanes, polyureas, poly(ethylene viny acetate),polypropylene, polymethacrylate, polyethylene, and poly(ethylene oxide).Copolymers, mixtures, and adducts of the above polymers may also be usedwith the invention.

In one embodiment, polyhydroxy acids such as polylactic acid (PLA),polyglycolic acid (PGA), and their copolymers (PLGA), (e.g.,poly(lactide-co-glycolide); 75/25), are used. These are among thesynthetic polymers approved for human clinical use as surgical suturematerials and in controlled release devices. They are degraded byhydrolysis to products that can be metabolized and excreted.Furthermore, copolymerization of PLA and PGA offers the advantage of alarge spectrum of degradation rates from a few days to several years bysimply varying the copolymer ratio of glycolic acid to lactic acid(which is more hydrophobic and less crystalline than PGA and degrades ata slower rate). In addition, the optical activity of poly(lactic acid)may be manipulated to control the degradation rate and other propertiesof the polymer. For example, poly(L-lactide) may be used alone or in acopolymer or mixture with poly(D,L-lactide), e.g.,poly(L-lactide-co-D,L-lactide). Exemplary ratios of L-lactide toD,L-lactide include 70/30.

Methods for using these polymers are well known. In general, thepolymers are dissolved in an organic solvent such as methylene chlorideor chloroform to mix with a mineral filler. The amount of solvent hasonly a minimal effect on the structure of the produced materials, butaffects the solvent evaporation time. Preferably, the solvent contains achlorine molecule, such as, for example, the solvents chloroform andmethylene chloride. The preferred solvent is chloroform.

Preparation of the Composite and Composite Processing

The composites of the invention may be reacted and then formed into thedesired shape or first formed or molded and then reacted into a fullycured state. Reaction may be achieved by thermal heating,electromagnetic heating or any other suitable means in the presence orabsence of catalysts. The incorporation of the bone particles into thepolymer matrix can be performed using one (or a combination) of thefabrication techniques known to those skilled in the art, such assolvent casting, melting, etc. The shaping of the inventive compositescan be carried out by any one of the following processes: compressionmolding, transfer molding, extrusion, injection molding, reactioninjection molding, sandwich molding, blow molding, extrusion blowmolding, injection blow molding, rotational molding, thermoforming,vacuum forming, machining, calendering, slush molding, lamination,spinning, etc.

Exemplary shapes include, but are not limited to, a sheet, plate,particle, sphere, strand, coiled strand, capillary network, film, fiber,mesh, disk, cone, rod, cup, pin, screw, tube, tooth, tooth root, bone orportion of bone, wedge or portion of wedge, cylinder, and threadedcylinder, In one embodiment, the composite is molded into the shape of adesired implant. For example, the mold may be shaped as a portion of abone or as a whole bone to be replaced. Bones that may be replaced usingthe composites of the invention include ethmoid, frontal, nasaloccipital, parietal, temporal, mandible, maxilla, zygomatic, cervicalvertebra, thoracic vertebra, lumbar vertebra, sacrum, rib, sternum,clavicle, scapula, humerus, radius, ulna, carpal bones, metacarpalbones, phalanges, ilium, ischium, pubis, femur, tibia, fibula, patella,calcaneus, tarsal and metatarsal bones. In one embodiment, the compositeis molded as a plate or similar support, including but not limited to anI-shape to be placed between teeth for intra-bony defects, a crescentapron for single site use, a rectangular bib for defects including boththe buccal and lingual alveolar ridges, neutralization plates, spoonplates, condylar plates, clover leaf plates, compression plates, bridgeplates, wave plates, etc. Partial tubular as well as flat composite maybe a block that is machined into a desired shape.

In another embodiment, a block of the composite material may be shaved,milled or ground, and the composite particles thus obtained may becombined in a mold having the desired shape or configuration, andpressed to form a solid as described in U.S. Pat. No. 6,294,187. Thecomposite particles may be mixed with additional biocompatiblecomponents, including biocompatible binders, fillers, fibers,plasticizers, biostatic/biocidal agents, surface active agents (e.g.,surfactants), biomolecules, small molecules, bioactive agents, etc,prior to, during, or after compression of the composite particles.

Wet-laying as described in U.S. Pat. No. 5,507,813 may also be used toassemble and form an implant from the particles of the inventivecomposite. In this technique, the composite particles are slurried in asuitable liquid and cast in a form such as a flat sheet, mesh screen, ora three-dimensional mold. The wet-laid mass is then dried by removingthe liquid by vacuum or evaporation. This process results in particleentanglement that provides the final implant with the ability to retainits shape. Further adhesion between the composite particles may beachieved by including an adhesive in the liquid or by using ultrasonicbonding. Additionally, the liquid may include biocompatible components.

In an alternative embodiment, the completed composite of the inventionis melted and molded into a desired shape. Thermoplastic polymers willflow upon heating and may be reshaped without machining. The polymer maybe rolled or extruded to form a particular shape or molded in the shapeof a desired implant, as discussed above. In an alternative embodiment,the composite is at least partially melted and inserted into an implantsite before cooling.

Incorporation of Other Materials Including Biologically-Active Agent

The composites of the invention are useful as stand alone materials, butthey can also be combined with other substrate materials to modify theirproperties. Thus, an important advantage of the inventive compositeslies in their ability to function as a carrier for, and effectivelyincorporate, one or more useful substances. These substances can bebiologically active or non biologically active compounds.

Biologically Active Substances. In one embodiment, the substancesincorporated into the composite of the invention promote new bone growthand connective tissue generation and/or accelerate wound healing (see,for example; U.S. Pat. No. 5,073,114). Examples of materials that can beincorporated include antibiotics, chemotherapeutics and bone cellinducers and stimulators, including the general class of cytokines suchas the TGF-β superfamily of bone growth factors [“Cytokines and BoneMetabolism” Gowen, ed, CRC press (1992)], the family of bonemorphogenetic proteins, osteoinductors, and/or bone marrow or boneforming precursor cells, isolated using standard techniques. Sources andamounts of various materials that can be included are known to thoseskilled in the art [Glowacki et al. “The role of osteocalcin inosteoclast differentiation” J. Cellular Biochem. 45:292-302 (1991);Ballock et al. “Regulation of collagen expression in periosteal cells bythree members of the TGF-β superfamily” Thirty Ninth Annual Meeting,Orthopaedic Research Society; 18,734 (1993); Ripamonti et al. “Inductionof bone in composites of osteogenin and porous hydroxyapatite inbaboons” J. Plastic and Reconstructive Surg. 89:731-739 (1991);Ripamonti et al. “Growth and morphogenetic factors in bone induction:role of osteogenin and related bone morphogenetic proteins” CRC CriticalReviews in Oral Biol. Med. 3:1-14 (1992); Ripamonti et al. “Initiationof bone regeneration in baboons by osteogenin, a bone morphogeneticprotein” Matrix; 12:40-55 (1992); Ripamonti et al. “Xenogeneicosteogenin and demineralized bone matrices including human induced bonedifferentiation in athymic rats and baboons” Matrix 11:404-411 (1991);Cook et al. “Restoration or large diaphyseal segmental defects inrabbits using recombinant human osteogenic protein (OP-1)” Combinedmeetings of Orthopaedic Research societies of USA, Japan and Canada 1,66 (1991); Miyamoto et al. “Trans-filter bone induction in monkeys bybone morphogenetic protein” Thirty Ninth Annual Meeting, OrthopaedicResearch Society 18, 99 (1993); Yasko et al. “Comparison of recombinanthuman BMP-2 versus cancellous bone to heal segmental bone defects”Thirty Ninth Annual Meeting, Orthopaedic Research Society 18, 100(1993); Aspenberg et al. “Bone morphogenetic protein induces bone in thesquirrel monkey, but bone matrix does not” Thirty Ninth Annual Meeting,Orthopaedic Research Society 18, 101 (1993); Iwasaki et al. “Bonemorphogenetic protein-2 stimulates osteogenesis in high density cultureof periosteum-derived cells” Thirty Ninth Annual Meeting, OrthopaedicResearch Society 18, 483 (1993); Cook et al. “Recombinant humanosteogenic protein-1 (rhOP-1) heals segmental long-bone defects innon-human primates” Thirty Ninth Annual Meeting, Orthopaedic ResearchSociety 18, 484 (1993); and Hunt et al. “Healing of a segmental defectin the rat femur using a bone inducing agent (BIA) derived from acultured human osteosarcoma cell line (SAOS-2)” Thirty Ninth AnnualMeeting, Orthopaedic Research Society 18, 489 (1993)].

To enhance biodegradation in vivo, the composites of the presentinvention can also include different enzymes. Preferred enzymes orsimilar reagents are proteases or hydrolases with ester-hydrolyzingcapabilities. Such enzymes include, but are not limited to, proteinaseK, bromelaine, pronase E, cellulase, dextranase, elastase, plasminstreptokinase, trypsin, chymotrypsin, papain, chymopapain, collagenase,subtilisn, chlostridopeptidase A, ficin, carboxypeptidase A, pectinase,pectinesterase, an oxidoreductase, an oxidase or the like. The inclusionof an appropriate amount of such a degradation enhancing agent can beused to regulate implant duration.

Suitable biologically-active agents also include substances useful inpreventing infection at the implant site, as for example, antiviral,antibacterial, antiparasitic, antifungal substances and combinationsthereof. The agent may further be a substance capable of acting as astimulant, sedative, hypnotic, analgesic, anticonvulsant, and the like.Anti-inflammatory compounds embedded within the composite will controlthe cellular response long after the initial response to implantation ofthe composite.

Inventive compositions may alternatively or additionally be used todeliver other pharmaceutical agents including antibiotics,anti-neoplastic agents, growth factors, hematopoietic factors,nutrients, etc. Bioactive agents that can be delivered using theinventive composites include non-collagenous proteins such asosteopontin, osteonectin, bone sialo proteins, fibronectin, laminin,fibrinogen, vitronectin, trombospondin, proteoglycans, decorin,proteoglycans, beta-glycan, biglycan, aggrecan, veriscan, tanascin,matrix gla protein hyaluran, cells; amino acids; peptides; inorganicelements; inorganic compounds; organometallic compounds; cofactors forprotein synthesis; cofactors for enzymes; vitamins; hormones; solubleand insoluble components of the immune system; soluble and insolublereceptors including truncated forms; soluble, insoluble, and cellsurface bound ligands including truncated forms; chemokines,interleukines; antigens; bioactive compounds that are endocytosed;tissue or tissue fragments; endocrine tissue; enzymes such ascollagenase, peptidases, oxidases, etc; polymeric cell scaffolds withparenchymal cells; angiogenic drugs, polymeric carriers containingbioactive agents; encapsulated bioactive agents; bioactive agents intime-release form; collagen lattices, antigenic agents; cytoskeletalagents; cartilage fragments; living cells such as chondrocytes,osteoblasts, osteoclasts, fibroclasts, bone marrow cells, mesenchymalstem cells, etc; tissue transplants; bioadhesives; bone morphogenicproteins (BMPs), transforming growth factors (TGF-beta), insulin-likegrowth factor (IGF-1, IGF-2), platelet derived growth factor (PDGF);fibroblast growth factors (FGF), vascular endothelial growth factors(VEGF), epidermal growth factor (EGF), growth factor binding proteins,e.g., insulin-like growth factors (IGFBP-2, IGFBP-3, IGFBP-4, IGFBP-5,IGFBP-6); angiogenic agents; bone promoters; cytokines; interleukins;genetic material; genes encoding bone promoting action; cells containinggenes encoding bone promoting action; cells genetically altered by thehand of man; externally expanded autograft or xenograft cells; growthhormones such as somatotropin; bone digestors; anti-tumor agents;fibronectin; cellular attractants and attachment agents;immunosuppressants; bone resorption inhibitors and stimulators;mitogenic factors; bioactive factors that inhibit and stimulate secondmessenger molecules; cell adhesion molecules, e.g., cell-matrix andcell-cell adhesion molecules; secondary messengers; monoclonalantibodies specific to cell surface determinants on mesenchymal stemcells; portions of monoclonal antibodies specific to cell surfacedeterminants on mesenchymal stem cells; portions of monoconal antibodiesspecific to cell surface determinants on mesenchymal stem cells;clotting factors; polynucleotides; and combinations thereof. The amountof the bioactive agent included in the composite can vary widely andwill depend on such factors as the agent being delivered, the site ofadministration, the patient's physiological condition, etc. The optimumlevels will be determined in a specific case based upon the intended useof the implant.

For example, inventive composites may be prepared so that they includeone or more compounds selected from the group consisting of drugs thatact at synaptic and neuroeffector junctional sites (e.g., acetylcholine,methacholine, pilocarpine, atropine, scopolamine, physostigmine,succinylcholine, epinephrine, norepinephrin, dopamine, dobutamine,isoproterenol, albuterol, propanolol, serotonin); drugs that can act onthe central nervous system (e.g., clonazepam, diazepam, lorazepam,benzocaine, bupivacaine, lidocaine, tetracaine, ropivacaine,amitriptyline, fluoxetine, paroxetine, valproic acid, carbamazepine,bromocriptine, morphine, fentanyl, naltrexone, naloxone); drugs that canmodulate inflammatory responses (e.g., aspirin, indomethacin, ibuprofen,naproxen, steroids, cromolyn sodium, theophylline); drugs that affectrenal and/or cardiovascular functions (e.g., furosemide, thiazide,amiloride, spironolactone, captopril, enalapril, lisinopril, diltiazem,nifedipine, verapamil, digoxin, isordil, dobutamine, lidocaine,quinidine, adenosine, digitalis, mevastatin, lovastatin, simvastatin,mevalonate); drugs that affect gastrointestinal function (e.g.,omeprazole, sucralfate); antibiotics (e.g., tetracycline. clindamycin,amphotericin B, quinine, methicillin, vancomycin, penicillin G,amoxicillin, gentamicin, erythomycin, ciprofloxacin, doxycycline,acyclovir, zidovudine (AZT), ddC, ddI, ribavirin, cefaclor, cephalexin,streptomycin, gentamicin, tobramycin, chloramphenicol, isoniazid,fluconazole, amantadine, interferon); anti-cancer agents (e.g.,cyclophosphamide, methotrexate, fluorouracil, cytarabine,mercaptopurine, vinblastine, vincristine, doxorubicin, bleomycin,mitomycin C, hydroxyurea, prednisone, tamoxifen, cisplatin,decarbazine); immunomodulatory agents (e.g., interleukins, interferons,GM-CSF, TNF-β, cyclosporin, FK506, azathioprine, steroids); drugs actingon the blood and/or the blood-forming organs (e.g., interleukins, G-CSF,GM-CSF, erythropoietin, vitamins, iron, copper, vitamin B₁₂, folic acid,heparin, warfarin, coumarin); hormones (e.g., growth hormone (GH),prolactin, luteinizing hormone, TSH, ACTH, insulin, FSH, CG,somatostatin, estrogens, androgens, progesterone, gonadotropin-releasinghormone (GnRH), thyroxine, triidothyronine); hormone antagonists; agentsaffecting calcification and bone turnover (e.g., calcium, phosphate,parathyroid hormone (PTH), vitamin D, bisphosphonates, calcitonin,fluoride); vitamins (e.g., riboflavin, nicotinic acid, pyridoxine,pantothenic acid, biotin, choline, inositol, camitine, vitamin C,vitamin A, vitamin E, vitamin K); gene therapy agents (e.g., viralvectors, nucleic-acid-bearing liposomes, DNA-protein conjugates,anti-sense agents); or other agents such as targeting agents, etc.

Non Biologically Active Agents. Non biologically active materials mayalso be incorporated into the inventive composites. For example,radiopaque (see, for example, U.S. Pat. No. 5,676,146), luminescent, ormagnetically active particles may be used. As the bone is resorbed,these non-biodegradable materials are removed from the tissue site bynatural metabolic processes, allowing the degradation of the polymer andthe resorption of the bone particles to be tracked using standardmedical diagnostic techniques. The composites of this invention mayfurther contain other materials such as fillers to improve the strengthof the polymer matrices, anti-degradants such as anti-oxidants andanti-ozonants, colorants, chromophores or any other material that mayalter or change the property of the composites.

Incorporation of Other Materials. In certain embodiments, the agent tobe delivered is adsorbed to or otherwise associated with the composite.The agent may be associated with the composite through specific ornon-specific interactions; or covalent or non-covalent interactions.Examples of specific interactions include those between a ligand and areceptor, an epitope, and an antibody, etc. Examples of non-specificinteractions include hydrophobic interactions, electrostaticinteractions, magnetic interactions, dipole interactions, van der Waalsinteractions, hydrogen bonding, etc.

Generally, the substances to be added to the composite can be chemicallyor physically bond to the polymer matrix or to the bone particles beforeformation of the composite. In that case, the agents to be added arepreferably either insoluble or substantially insoluble in the leachingmedia. As discussed above, biologically active and non-biologicallyactive compounds can be linked to the bone particles through the silanecoupling agents. The substances can also be added after formation of thecomposite by standard dip or spray application techniques followed bydrying. Alternatively, after removal of the pore-forming agent, thecomposite can be treated with reagents that generate functional groupsin the polymeric matrix to which biologically active or non biologicallyactive agents can be chemically or physically attached. In certainembodiments, the agent is attached to the matrix using a linker so thatthe agent is free to associate with its receptor or site of action invivo. In certain preferred embodiments, the agent to be delivered may beattached to a chemical compound such as a peptide that is recognized bythe matrix of the composite. In another embodiment, the agent to bedelivered is attached to an antibody, or fragment thereof, thatrecognizes an epitope found within the matrix of the composite. In apreferred embodiment, the agent is BMP, TGF-β, IGF, parathyroid hormone(PTH), growth factors, or angiogenic factors. In certain embodiments, atleast two bioactive agents are attached to the composite. In otherembodiments, at least three bioactive agents are attached to thecomposite.

Preferably, the site, where the biologically active or non biologicallyactive agents are attached to in the composite, are biodegradable sothat the agents can be released to the adjacent tissue fluids duringbiodegradation of the matrix. Preferably, agents are released into thesurrounding tissue fluids at a controlled rate. For example, the polymermatrix may be formulated to degrade after an effective and/orsubstantial amount of the agent is released from the matrix. Release ofa substance having a low solubility in water, as for example, a peptideor protein, may require the degradation of a substantial part of thepolymer matrix to expose the agent directly to the surrounding tissuefluids. Thus, the release of the agent from the matrix may be varied by,for example, the solubility of the agent in water, the distribution ofthe agent within the matrix, or the size, shape, porosity, solubilityand biodegradability of the polymer matrix.

Treatments of the Implant

Once the composite of the invention has been shaped into an implant, itcan be used as such or further processed. The goal of these furthertreatments is to modify the properties of the implant, such as its rateof degradation or its ability to promote bone growth, and/or to changethe shape of the implant in order to broaden the range of its potentialclinical applications.

For example the surface of the implant can be oxidized using a solventor gas to break some of the polymer chains and thereby accelerate theinitial decomposition of the implant. The surface of the implant canalso be roughened to promote bony on-growth. This can be achieved bysanding, filing, plasma etching, chemically etching, or mechanicallypitting. Different procedures aimed at attaching biologically active andnon biologically active compounds to the inventive implants have beendescribed above. In addition to these procedures, the surface of thecomposite can be submitted to plasma etching or chemical oxidation torender the implant more reactive and increase its affinity for the agentto be attached to it (see U.S. Pat. Nos. 6,033,582 and 6,119,028).

The implant can also be machined according to techniques well known inthe art. For example, holes may be drilled to facilitate bony ingrowthor to provide channels for suturing tissues to the implant.Alternatively, a composite shaped as a block can be machined into adesired shape. These machined components may be attached to one anotherusing mechanical fasteners such as dowels, pins, and screws, all ofwhich may be fabricated from the composite of the invention. Traditionaljoints such as tongue-and-groove or mortoise-and-tenon may be employedas the machined pieces are assembled.

Alternatively, or in addition, the machined pieces may be attached toone another, by using a biocompatible adhesive or a chemicalcross-linking agent or by ultrasonic bonding. Biocompatible adhesivesinclude, but are not limited to, biocompatible cyanoacrylates,epoxy-based compounds, dental resin sealants, dental resin cements,glass ionomer cements, poly(methyl methacrylate),gelatin-resorcinol-formaldehyde glues, collagen-based glues, inorganicbonding agents such as zinc phosphate, magnesium phosphate, and otherphosphate based cements, zinc carboxylate, and protein-based binders,such as fibrin glues and mussel-derived adhesive proteins.

Additional Applications

Non weight bearing applications. In another embodiment, the inventionprovides bone substitute materials that can be used for non load bearingpurposes. For example, a flowable material for filling defects incancellous bone such as vertebral bodies might benefit from a stronginteraction between the bone particles and the fluid or gel phase. Thiscan prevent settling and improve the lubrication and flow properties ofthe material in order to make injection easier. Depending on thecomposition of the fluid or gel phase and the nature of the R group onthe silane, the interaction may be direct or indirect covalent ornon-covalent interaction. Desirable non-covalent interactions includehydrogen bonding, van der Waals interactions, hydrophobic interactions,electrostatic interactions, etc.

Other Preparation Methods. In addition to bone particles, the methods ofthe invention can be applied to constructs that incorporate large piecesof bone. A coupling agent can be used to bind a polymeric surfacecoating to a monolithic bone piece.

Several methods are known in the art to bond machined composite piecesto one another, these include, for example, application of known andconventional biologically compatible adhesives or addition ofbiocompatible chemical cross-linking agents, use of mechanicalfasteners, which can be fabricated from natural and synthetic materialsand bioabsorbable as well as nonbioabsorbable materials, laser tissuewelding, and, ultrasonic bonding. The inventive method provides a newway to efficiently bond columns of bone and bone-derived compositetogether to form a weight bearing implant.

As described in U.S. Pat. No. 5,899,939, the final bone-derived implantcan optionally possess one or more layers formed from one or more othermaterials. For example, these optional layers can be based on or includehighly or fully demineralized bone, graphite or pyrolytic carbon, amineral material such as hydroxyapatite, tricalcium phosphate, bioglassor other bioceramic, or natural or synthetic polymers.

1. A composite, comprising: a plurality of bone particles and abiocompatible polymer, wherein at least a portion of the bone particlesare covalently linked to the polymer through a silane coupling agent,wherein the bone particles represent about 60% to about 75% of the totalweight of the composite.
 2. A composite comprising: a plurality of boneparticles; and a biocompatible polymer; wherein at least a portion ofthe bone particles are covalently linked to the polymer through a silanecoupling agent; and wherein the polymer is selected from the groupconsisting of polysaccharides.
 3. The composite of claim 2, wherein thepolymer is selected from the group consisting of starch, dextran,cellulose, gelatin, collagen, and derivatives thereof.
 4. The compositecomprising: a plurality of bone particles; a biocompatible polymer; anda cross-linking agent; wherein at least a portion of the bone particlesare covalently linked to the polymer through a silane coupling agent;and wherein the cross-linking agent is selected from the groupconsisting of aldehydes, polyepoxy compounds, polyvalent metallicoxides, organic tannins, N-hydroxysuccinimides,N-hydroxysulfosuccinimides, phenolic oxides, hydrazides, carbodiimides,isocyanates, isothiocyanates, sugars, and enzymes.
 5. The composite ofclaim 1, 2, or 4, wherein the bone particles are obtained from one ormore of autologous bone, allogenic bone, xenogenic bone, or mixturesthereof.
 6. The composite of claim 1, 2, or 4, wherein the boneparticles are obtained from one or more of cortical bone, cancellousbone, cortico-cancellous bone, or mixtures thereof.
 7. The composite ofclaim 1, 2, or 4, wherein the bone particles are obtained from one ormore of nondemineralized bone, deorganified bone, anorganic bone, ormixtures thereof.
 8. The composite of claim 2 or 4, wherein the boneparticles represent about 60% to about 75% of the total weigh of thecomposite.
 9. The composite of claim 1, 2, or 4, wherein the couplingagent is a silane selected from the group consisting of silanes bearingone hydrolyzable or leaving group, silanes bearing two hydrolyzable orleaving groups, and silanes bearing three hydrolyzable or leavinggroups.
 10. The composite of claim 1, 2, or 4, wherein the polymer is abiocompatible polymer selected from the group consisting of polymers ofnatural origin, polymers of artificial origin, and any combination ofnatural and artificial polymers.
 11. The composite of claim 1, 2, or 4,wherein the polymer is a selected from the group consisting ofbiodegradable polymers, non-biodegradable polymers, co-polymers ofbiodegradable polymers, co-polymers of non-biodegradable polymers, andco-polymers of biodegradable and non-biodegradable polymers.
 12. Thecomposite of claim 1 or 4, wherein the polymer is a natural polymerselected from the group consisting of polysaccharides.
 13. The compositeof claim 8, wherein the polymer is selected from the group consistingstarch, dextran, cellulose, gelatin, collagen, and derivatives thereof.14. The composite of claim 1 or 4, wherein the polymer is an artificialpolymer selected from the group consisting of poly(anhydrides),poly(hydroxy acids), polyesters, poly(orthoesters), polycarbonates,poly(propylene fumerates), poly(caprolactones), polyamides, polyaminoacids, polyacetals, polylactides, polyglycolides, poly(dioxanones),polysulfones, polyhydroxybutyrates, polyhydroxyvalyrates, poly(vinylpyrrolidone), biodegradable polycyanoacrylates, biodegradablepolyurethanes, polysaccharides, tyrosine-based polymers, poly(methylvinyl ether), poly(maleic anhydride), poly(glyconates),polyphosphazines, poly(esteramides), polyketals, poly(orthocarbonates),poly(maleic acid), poly(alkylene oxalates), poly(alkylene succinates),poly(pyrrole), poly(aniline), poly(thiophene), polystyrene,non-biodegradable polyurethanes, polyureas, poly(ethylene vinylacetate), polypropylene, polymethacrylate, polyethylene, poly(ethyleneoxide), and co-polymers, adducts, and mixtures thereof.
 15. Thecomposite of claim 1, 2, or 4, wherein a surface of at least a portionof the bone particles has been chemically modified.
 16. The composite ofclaim 15, wherein the surface of at least a portion of the boneparticles is treated with dilute phosphoric acid.
 17. The composite ofclaim 1, 2, or 4, wherein the composition of the bone particle has beenmodified.
 18. The composition of claim 17, wherein at least a portion ofthe bone particles are dried, lyophilized, defatted, treated with adetergent, treated with a solvent, treated with a surfactant, or treatedto remove or inactivate pathogens.
 19. The composite of claim 1 or 2,further comprising a cross-linking agent.
 20. The composite of claim 19,wherein the cross-linking agent is selected from the group consisting ofaldehydes, polyepoxy compounds, polyvalent metallic oxides, organictannins, N-hydroxysuccinimides, N-hydroxysulfosuccinimides, phenolicoxides, hydrazides, carbodiimides, isocyanates, isothiocyanates, sugarsand enzymes.
 21. The composite of claim 1, 2, or 4, further comprisingone or more of a wetting agent, biocompatible binder, filler, fiber,plasticizer, biostatic/biocidal agent, surface active agent,biomolecule, small molecule, or bioactive agent.
 22. The composite ofclaim 21, wherein the bioactive agent is selected from the groupconsisting of antibiotics, chemotherapeutics, bone cell inducers, andbone cell stimulators.
 23. The composite of claim 1, 2, or 4, furthercomprising osteoblasts.
 24. The composite of claim 21, wherein thebioactive agent is selected from the group consisting of bonemorphogenic proteins (BMPs), transforming growth factor-beta (TGF-β),insulin-like growth factor, platelet-derived growth factor (PDGF),vascular endothelial growth factor (VEGF), epidermal growth factor(EGF), parathyroid hormone (PTH), growth factor binding proteins, andangiogenic factor.